This project aims to develop a commercial software product, VirtualDoseTM, for calculating and analyzing patient radiation doses from x-ray computed tomography (CT) examinations. Despite its essential role in radiology, CT is responsible for raising the collective medical radiation dose to the American population to a level that is associated with measurable and predictable risk for carcinogenic effect today. Accurate knowledge and record of patient organ doses have now become critical in the successful and sustainable use of this imaging modality. It is recognized that existing software are insufficient in meeting the current and future needs involving modern MDCT scanners. Based on unique virtual patient technologies and extensive Monte Carlo modeling capabilities and modern software engineering, this proposed software system is expected to improve upon the accuracy and functionality in CT dose estimates. Specific aims of the project are: (1) To adopt a family of anatomically realistic phantoms including the adult male/female, pregnant female, and male/female children with the capability to adjust the body size and shape. (2) To develop a Monte Carlo based computational method for modeling various CT scanners and clinical protocols. (3) To establish a comprehensive organ dose database by using the Monte Carlo CT scanner models and ICRP algorithms. (4) To develop and refine the software graphical user interfaces by using 3D object- oriented programming.